Molecular Biology Methods: PCR, Cloning, Sequencing, and Blotting
Core Experimental Techniques
Why These Methods Matter
Molecular biology depends on tools that can amplify nucleic acids, detect specific sequences, clone DNA into vectors, and read genome-wide information. These methods support diagnosis, research, biotechnology, and genetic engineering.
Technique Overview
| Method | Main Use |
|---|---|
| PCR | Amplify specific DNA regions |
| Gel electrophoresis | Separate nucleic acids by size |
| Cloning | Propagate or express a DNA insert |
| Sequencing | Determine nucleotide order |
| Blotting | Detect specific DNA, RNA, or protein |
PCR Logic
PCR relies on repeated cycles of denaturation, annealing, and extension to produce exponential amplification of a target sequence.
What is the purpose of primers in PCR?
DNA polymerase needs a preexisting 3' end to extend from.
Correct answer: To provide a starting point for DNA polymerase
Why is Sanger sequencing useful?
Chain termination lets researchers determine nucleotide order with high precision.
Correct answer: It provides accurate sequence information for relatively short DNA fragments.
Typical PCR Workflow
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1
Step 1: Prepare template DNA, primers, polymerase, nucleotides, and buffer.
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2
Step 2: Denature the DNA strands.
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3
Step 3: Anneal primers to complementary target sequences.
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4
Step 4: Extend new strands using a thermostable polymerase.
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5
Step 5: Repeat cycles to amplify the target exponentially.
Blotting Methods
Southern blot
- Detects DNA
- Uses labeled probes
Northern blot
- Detects RNA
- Assesses transcript size and abundance
Which experimental result most strongly suggests successful cloning into a plasmid?
Selection and screening together support successful recombinant cloning.
Correct answer: A selectable marker and insert-specific screening both indicate the insert is present
What does gel electrophoresis separate nucleic acids by?
Smaller fragments generally migrate farther through the gel matrix.
Correct answer: Size, and to a lesser extent conformation and charge effects.
Which method would you use to measure transcript abundance for a known gene?
Northern blot and RT-qPCR are standard transcript-detection methods.
Correct answer: Northern blot or RT-qPCR